The present invention relates to an optical disk in which signals are recorded op both of groove tracks and land tracks formed between groove tracks, and a method of tracking such an optical disk.
Technical developments for large-capacity read-only optical disks indicate that they will be widely used as digital video disks and computer data ROM disk. Also, the density of rewritable optical disks is steadily increasing in 12 cm diameter rewritable optical disks which currently have a capacity 4 times that of conventional CDs (compact dics), therefore realization of a large-capacity disk of 2.6 GB per side is drawing near.
The technology for increasing recording density of rewritable disks will be described first. Currently, the land/groove (L/G) recording format is being studied as one of the rewritable optical disk signal recording formats. FIG. 3 shows the structure of inscribed land/groove tracks for conventional optical disks. As disclosed in Japanese Examined Patent Publication No. 63-57859, the recording pits (recording marks) are formed both in grooves 94 formed in the record layer 91, and lands 95 between the grooves 94. Numeral 93 denotes a light spot for recording and reproducing information signal.
Compared with conventional optical disks in which recording pits are formed in the grooves 94 only, the L/G recording configuration can record twice the data, the track pitch can be halved when a disk of the same groove pitch as conventional optical disks is used. In addition, in enhancing the density of optical disks, the L/G recording configuration has the following advantages.
First, the groove width is substantially equal to that of conventional optical disks, and the servo error signal can be picked up as a high fidelity signal as in continuous groove tracking or continuous land tracking, so reliability of apparatus operation increase. Secondly, as described in "Land and Groove Recording for High Track Density on Phase-Change Optical Disks" by N. Miyagawa, et al. (Jpn-J-Appl. Phys., vol. 32, pp. 5324-5328, 1993), by taking advantage of reduced crosstalk resulting from an optimized groove depth, a narrower recording track pitch is achieved. However, problems associated with L/G recorded optical disks are that the tracking polarity must be switched between groove and land during tracking, and an additional circuit is needed for jumping during tracking. These problems, however, can be overcome by a minor change in the conventional tracking circuit for continuous groove tracking.
One unsolved problem for the L/G recording is a track format which allows the groove track data and land track data to be joined so as to enable continuous access. A track format suitable for continuous access is essential for video disks, while at the same time the high-speed random access of data disk files must also be supported.
Two specific examples of track formats for L/G recording are given below.
FIG. 4 shows a general configuration of track format having groove and land tracks connected every revolution to form a single recording spiral. The black region G shows groove tracks and the white region L shows the land tracks. This format is disclosed in Japanese Laid-Open Patent Publications No. 4-38633, No. 6-274896, No. 8-87777. The optical disk format of FIG. 4 will be called the single-spiral land/groove (SS-L/G) format in the present specification.
FIG. 5 shows the general configuration of another track format for conventional L/G recording format consisting of two separate recording spirals of the groove and land tracks. This recording format is known as a double spiral land/groove (DS-L/G) format. In both of the cases shown in FIG. 4 and FIG. 5, the disk configured in an aligned ID format in which tracks are divided into sectors (eight sectors in the example illustrated), each consisting of an information recording region D and an identification signal region I, for recording the address information of each sector. The identification signal regions are aligned in the radial direction. This configuration prevents crosstalk from the identification signals in the neighboring tracks during playback of information from the information recording region D (which would occur if the information recording region and the identification signal region I are adjacent to each other).
As shown in FIG. 4 for the SS-L/G format, there is a point at one location per revolution, where groove and land tracks are connected at the identification signal region IB.
Next, L/G recording track formats of FIG. 4 and FIG. 5 are compared.
The SS-L/G recording track format shown in FIG. 4 possesses a merit that the recording track is one continuous line on the disk which allows for continuous recording/reproduction of data and the radial access method similar to that of conventional CDs is applicable since data is sequentially arranged on a spiral. Furthermore, this formation is suitable for optical disk track formats of video files where continuous recording/reproducing capabilities are an indispensable requirement. For the conventional DS-L/G format shown in FIG. 5, land and groove tracks separately form independent spirals. A special track jump method is required to handle optical head transition from, for example, the end of the land track after one scan of the entire spiral to the start of the groove track. So, continuous recording/reproduction must be interrupted in at least one place due to connecting access between the land and track grooves. To avoid this interruption, a buffer memory may be added, but this raises the cost of related equipment. For the SS-L/G format, this buffer memory is not necessary.
However, as mentioned previously, the connecting point between groove and land recording tracks must be detected in order to determine whether to scan on a groove track or a land track, and the polarity of the tracking servo must be switched. That is, the tracking servo polarity must be switched correctly for every disk revolution. The L/G switching region detection is difficult, and accurate tracking control for data recording/reproduction has not been possible. For this reason, the SS-L/G format of the optical disk has not successfully been put into practical use.
Next, the prior identification signal insertion method of the DS-L/G recording format of the optical disk as shown in FIG. 5 will be described. Three methods of configuring the prepits in the identification signal region shown in FIGS. 6A to 6C are currently known. In FIGS. 6A to 6C, I is the identification signal region and D is the information recording region. The method shown in FIG. 6A is called the land/groove independent address method. Dedicated prepits are included for each sector of the land track and the groove track. For this format, if the identification signal pit width and groove width are the same, the prepits of the adjoined track sectors are connected and the signal cannot be detected. Therefore, the identification signal pit width and groove width must be different, and usually, the prepit width is half that of the groove.
However, for mastering process of mother stamper in disk fabrication, the cutting beam diameter must be changed to form grooves and to form prepits. More-specifically, a narrow prepit width means that two beams must be employed for cutting the mother stamper in mastering, one for groove cutting and one for pit cutting. Also, if the two beam centers are not aligned, a tracking offset will occur between reproduction of identification signal and recording/reproducing of information recording signal. This tracking offset degrades the quality of the playback data. To ensure reliability of the playback data, a highly accurate positioning of the two beams is required during original disk fabrication, which increases disk costs.
In view of disk production accuracy and cost, the format shown in FIG. 6B and FIG. 6C in which the same beam can be used for grooves and pits is preferred over the format shown in FIG. 6A. In the format shown in FIGS. 6B and 6C, the width of the prepits forming the identification signal is substantially equal to the width as the grooves.
The method shown in FIG. 6B is called the land/groove &gt;shared address format. The prepits are placed on an extension of a boundary between a given pair of adjacent groove and land tracks, so each track shares the same identification signal. Details are disclosed in Japanese Laid-Open Patent Publication No. 6-176404.
FIG. 6C shows the time-division L/G independent address format as disclosed in Japanese Laid-Open Patent Publication No. 7-110944. An identification signal is included as address information independently for each of the land and groove tracks. For this format, the identification signals in the header section in tracks adjacent to each other are shifted in the direction of the tracks, so the identification signals in the adjacent tracks are not aligned in the radial direction. A drawback of this format is a low efficiency of the sector format due to the double length of identification signal regions.
Next, the rotation control method of the L/G record optical disk is described.
For optical disk track formats of video files where continuous recording/reproducing capabilities are indispensable, the rotational speed of the disk is changed so that the data rate is identical between inner and outer peripheries of the disk. Also, for rewritable optical disk, compatibility with conventional read-only optical disk is important, so use of phase change mediums is preferable. For this recording format, the disk recording layer is heated and changed by the light from crystalline phase to amorphous phase, and recording marks are thereby formed. The optical head for phase-change recording can be easily shared by the optical head for a read-only optical disk.
However, current phase-change mediums do not have sufficient recording/reproduction capabilities. Because the adaptable range of recording linear velocity during pulse width modulation (PWM) recording is narrow, the achievable recording/reproducing characteristic is not satisfactory. A detailed description of the recording/reproducing characteristics is given as follows. When the disk rotational speed is controlled in CAV (Constant Angular Velocity) control mode, and if the disk rotational speed is fixed to yield the required data rate for inner radial part of the disk, the recording linear velocity of outer radial part of the disk will be 2.5 to 3.0 times faster than that of inner radial part. Accordingly, the signal processing circuit must process data in the outer radial part at nearly 3 times higher than the data rate of in the inner radial part. Designing a low cost recording/reproducing apparatus which accommodates such a wide recording speed range is difficult. Moreover, for video applications, a constant data rate for inner and outer circumferences of the disk is desirable.
A solution is as follows. For a rewritable digital video disk having compatibility with ROM, requirements for disk media characteristics and data record/reproduction circuit performance call for employment of a ZCLV (Zoned Constant Linear Velocity) format. The ZCLV is a recording/reproducing format where the optical disk is divided into multiple annular zones, and the disk rotational speed is changed depending on the zones, so that the data transfer rate can be fixed and the linear velocity is substantially constant in all the zones. However, a problem with the rotation control system for an optical disk where the disk rotational speed changes is that the quick and correct detection of the groove-land connecting point is difficult because the periods between the groove-land connecting points vary during radial seek operation crossing zone boundaries.
As has been described, for rewritable optical disks employing conventional single-spiral L/G recording formats, it is difficult to achieve quick and correct detection of the land-groove connecting point, and a high level of reliability, while maintaining the recording density.